1. Field of the Invention
The present invention concerns a pigment dispersion and an ink composition using the pigment dispersion. More specifically, the present invention relates to a pigment dispersion excellent in the dispersibility and the storage stability, and an ink composition suitable to inkjet printing containing the pigment dispersion capable of forming images at high image quality, and curable by irradiation of actinic radiation rays.
2. Description of the Related Art
In a case of using a pigment as a colorant in various coloring compositions, it is important to ensure the dispersibility and the dispersion stability of the pigment as a solid. A curable composition or an ink composition having a uniform hue can be obtained by using a pigment dispersion excellent in pigment dispersibility and dispersion stability. Particularly, a pigment of excellent in light fastness is used generally as a colorant for a ink composition. However, in a case where the pigment involves a problem in view of the pigment dispersibility, this results in various problems, for example, of rendering the tone not uniform or lowering the dischargeability in a case of an ink composition used for an inkjet recording method.
In recent years, an inkjet recording method has attracted attention as an image recording method of forming images on a medium to be recorded based on image data signals. The inkjet recording method has advantages of generating less noises, and is also capable of recording highly fine images at a low running cost by hitting extremely small liquid droplets.
According to the inkjet recording method, while printing is possible not only to plain paper but also to a not-liquid absorbing medium to be recorded such as a plastic sheet or a metal plate, it is desired to shorten the time necessary for drying and curing for attaining higher speed and higher image quality upon printing. For this purpose, one of inkjet methods involves a recording method of using an ink that can be cured in a short time by irradiation of actinic energy rays as an inkjet recording ink. According to this method, clear and sharp images can be formed by irradiating actinic energy rays just after printing thereby curing ink droplets. For such a curable inkjet composition, high pigment dispersibility and aging stability thereof are demanded for forming highly fine images of excellent coloring property and stably discharging the ink composition.
For providing the ink composition with a clear tone and a high coloring power, it is essential that the pigment is made finely particulate. Particularly, in an ink composition used for inkjet recording, since discharged ink droplets give a significant effect on the clearness and sharpness of images, the amount of the discharged liquid droplets is decreased and it is essential to use particles which are finer than the thickness of the cured ink film formed from the ink composition. However, as the pigment particles are made finer for obtaining a higher coloring power, dispersion of fine particles becomes difficult tending to form aggregates. Further, this also results in a problem that the viscosity of the composition increases by excessive addition of the dispersing agent. Since the generation of pigment aggregates and increase of the viscosity of the ink composition gives an undesired effect on the ink dischargeability, it is not preferred to use an ink composition suffering from pigment aggregates or increase of viscosity for inkjet recording.
Further, in a case of using the ink composition as the inkjet recording, it is necessary that heat cycle characteristics are excellent. The ink composition used for inkjet recording is contained in a cartridge and heated upon discharge for lowering the liquid viscosity, and temperature is lowered in a discharge-rest period or during storage. Then the composition undergoes repetitive temperature change of heating and cooling. The change of temperature also gives an undesired effect on the pigment dispersibility to result in a problem of lowering the pigment dispersibility with time, thereby tending to cause aggregation of the pigment and increase of the viscosity.
A method of manufacturing an ink composition containing a fine pigment dispersion includes a method of previously preparing a concentrated pigment dispersion (also referred to as a mill base) and diluting the obtained mill base with a desired liquid such as a solvent or a polymerizable compound, thereby obtaining an ink composition. However, since the mill base is a concentrated pigment dispersion, interaction exerts strongly between pigments thereby tending to increase the viscosity during dispersion or storage. Increase of the viscosity of the mill base is not preferred since this increases the burden on a manufacturing machine, as well as results in lowering of the manufacturing stability due to circulation failure or lowering of the re-dispersibility upon ink preparation. With the view point described above, it has been demanded for a mill base having sufficient fluidity and storage stability, and an ink composition used for inkjet having sufficient fluidity in which a finely particulate pigment is dispersed uniformly and which is also excellent curability.
For the dispersing agent for obtaining a stable ink composition, various proposals have been made as described below. That is, an ink composition using a pigment derivative as a dispersing agent for improving the affinity with a pigment (for example, refer to Japanese patent application laid-open (JP-A) Nos. 2003-119414 and 2004-18656), an ink composition using a polymer having a basic group as a dispersing agent to a specified pigment such as a phthalocyanine or a quinacridone type (for example, refer to JP-A No. 2003-321628), an ink composition not using an organic solvent and containing a dispersing agent such as a poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer and a specified monomer for dissolving the dispersing agent (for example, refer to JP-A No. 2004-131589), or an ink composition using a graft copolymer having a heterocyclic residue that forms an organic pigment as a dispersing agent (for example, refer to JP-A No. 2007-9117) have been disclosed.
The pigment dispersion or the ink composition disclosed in the documents described above can indeed disperse the pigment finely and the stability of the ink composition is improved more than that in existent cases, but there is still a room for the improvement the stability of the ink composition. Further, also for an ink composition of excellent stability, improvement has been demanded for the stability of the mill base.